You want an honest answer? It means you dont have an interesting personality. You are maybe boring or irritating or nagging. It's like how girls say "you are too nice" which means that the guy is boring and a pushover. "too hot to date"- shit personality.
Sorry if it hurts, but I always prefer to hurt with the truth than to comfort with a lie
Because you're so hot that guys will be all over you, even if you're in a relationship. And that is very annoying and puts a lot insecurity in a man. You may think he should be flattered for having someone that others also want... but it gets really annoying to fast. Imagine if you dated DiCaprio or anyone you consider super hot, and you'll get the idea
It's not you that is the problem, it's that guys in your age range are often not interested in anything serious. They want to fuck you and maybe hang around for a while, but they're not looking to commit. Focus on other areas of your life for a while. I can promise you that dating in your late twenties is so much more satisfying. If something does happen to come along in the meantime, great! But don't make finding a boyfriend a major priority at this point.
Maybe these guys just don't want to be in relationships in general. Or maybe they're sweet-talking you so that they're more likely to get sex. Or maybe they don't feel compatible with you personality-wise.